The goal of the proposed research is to determine to what degree too large a socket impacts the health and quality of life of individuals with transtibial amputation using prosthetic limbs. The clinical relevance of this research is to promote quantitative standards for socket replacement based on prosthetic fit rather than duration of prosthesis use, as is current practice. The specific aims are to characterize volume changes typically made to transtibial sockets replaced because of residual limb volume reduction and to characterize differences in the daily residual limb volume change, gait stability, comfort, and satisfaction with the prosthesis, and activity level for individuals with transtibial amputation using enlarged sockets as opposed to their original as-prescribed sockets. To accomplish the aims, the volume differences between replacement and original sockets typically made for patients having their socket replaced because of residual limb volume reduction are determined. Automated fabrication methods are then used to make prosthetic test sockets that have known and quantified socket volume differences (relative to the original as-prescribed sockets) for a population of transtibial amputee subjects. After one-month wear periods, outcomes are assessed for each study participant. Bioimpedance analysis is used to assess daily residual limb fluid volume change, gait analysis to assess variability in walking patterns, subjective survey instruments to evaluate satisfaction and comfort with the prosthesis, a sock monitor to assess sock ply changes, and a portable gait monitor to assess overall activity. Hypotheses are evaluated to determine if the outcome variables are significantly different when patients wear the enlarged test sockets compared with the original as-prescribed sockets. The relevance of the proposed application to public health is better understanding of how socket fit impacts user experience with a prosthesis, clinically-important outcomes related to residual limb health, and performance measures traditionally measured in prosthetics research. Currently, it is unknown how poorly fitting sockets impact daily limb volume change, functional performance, satisfaction with the prosthesis, comfort, sock ply change, and activity level of individuals with limb amputation. Beyond the grant period, it is anticipated the proposed research will be extended into clinical tools that allow practitioners to isolate socket design problems from other issues such as improper componentry or alignment. Potentially, the efforts described here can be extended to other areas including trans-femoral prosthetics, upper-limb prosthetics, and orthotics.